1. Field of the Invention
The present invention relates to an optical disk which is a medium for recording and storing information, an optical disk device which is used for recording and/or reproducing information on such an optical disk, a data recording method, and a data reproducing method.
2. Related Art of the Invention
The conventional art will be described with reference to FIGS. 19 to 22.
FIG. 19 shows a cross-section configuration of an optical disk device of the conventional art. Referring to FIG. 19, light beams 2 emitted from a radiation light source 1 such as a semiconductor laser are transmitted through a beam splitter 3, and then converted into parallel beams 5 by a collimating lens 4. The light beams 5 are reflected by a reflecting mirror 9 and then converged by an objective lens 11 onto a signal surface 12S on a track formed in the rear face of a optical disk substrate 12.
The objective lens 11 is controlled by an actuator in focusing and tracking directions with respect to the signal surface 12S. The light beams reflected from the signal surface 12S are converged by the objective lens 11 into light beams 15 which are then reflected by the reflecting mirror 9, passed through the collimating lens 4, reflected by the beam splitter 3, and then incident on a photo detector 16. The photo detector 16 detects the amount of a spot 15S of the light beams 15 to produce a sum signal 41S. An actual optical disk device further comprises a configuration for detecting a focussing error and a tracking error. However, such a configuration is not directly related to the present invention, and therefore its description is omitted.
FIGS. 20 and 21 show a cross-section configuration of the optical disk. Referring to FIG. 20, grooves 12G and lands 12L which are concaved and convexed are formed in the signal surface 12S of the optical disk at a pitch p in a radial direction of the optical disk substrate 12. Signal marks 20 having a complex reflectance which is different from that of another region are formed on each of the grooves 12G. Differences of the reflectances are read as a reproduced signal by means of a converged light spot 21 which is subjected to scanning along the groove 12G.
Alternatively, an optical disk may have the cross-section configuration of FIG. 21. Referring to FIG. 21, grooves 12G and lands 12L which are concaved and convexed are formed in the signal surface 12S of the optical disk at a pitch 2p in a radial direction of the optical disk substrate 12. Signal marks 20 having a complex reflectance which is different from that of another region are formed on each of the grooves 12G and the lands 12L. Differences of the reflectances are read as a reproduced signal by means of a converged light spot 21 which is subjected to scanning along the groove 12G and the land 12L.
FIG. 22 is a diagram showing the principle of generation of binary codes in the conventional art example. Referring to FIG. 22, when signal marks 22 and 23 are scanned by the converged light spot 21, a sum signal 41S has a signal waveform 24. The signal waveform 24 is sliced at an appropriate detection level 25, so that slice points 24a, 24b, 24c, and 24d are detected at positions corresponding to the beginnings and ends 22S and 22E, and 23S and 23E of the signal marks 22 and 23.
It is assumed that code 1 is set at a bit which is immediately after a slice point and code 0 is set at any bit other than such a bit. If the basic unit T (1 bit length) of the signal length is T=0.14 μm, binary codes which are read from the signal waveform 24 are 10010010001. . . . In this way, signal marks are converted into binary codes.
Such an optical disk and an optical disk device of the conventional art have the following problem in increasing the information density. When the density of signal marks on an optical disk is increased while fixing the diameter of the converged light spot, crosstalk or interference between codes is increased, thereby impairing the quality of the signal. When aberration and variations of the optical system including the disk are taking into account, there is little room for increasing the density.